Protective coating for ferrous metals

ABSTRACT

Articles of ferrous metal protectively coated with a layer of a metal more noble than the ferrous metal and with an electricallyconductive overlayer formed by drying and heat curing to water insolubility a composition comprising aluminum powder in an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or metal salts of said acids.

United States Patent 11 1 1111 3,897,222

Hood 1 July 29, 1975 [54] PROTECTIVE COATING FOR FERROUS 3,113,845 12/1963 Uchida 29/1966 X METALS 3,248,251 4/1966 Allen 106/286 3,295,936 1/1967 Asano 29/1966 X Inventor: Andrew Cralg d, yne, a- 3,298,936 1/1967 Michael 29/195 M x [73] Assignee: Standard Pressed Steel Co., gs g Jenkimown Pa [22] Filed: Sept 1973 Primary ExaminerL. Dewayne Rutledge [21] Appl. No.: 398,578 Assistant Examiner-Arthur J. Steiner Related U.S. Application Data [63] Continuation of Ser. No. 149,360, June 2, 1971, [57] ABSTRACT abandoned.

Articles of ferrous metal protectively coated with a [52] U.S. Cl. 29/195; 29/196; 29/1966; layer Ofa meta] mere noble than the ferrous metal and 7/71 M with an electrically-conductive overlayer formed by [51] Int. Cl. B32b 15/00; C23b 5/04 drying and heat curing to water insolubmty a composi [58] Field of Search 1 17/71 M, 62, 217; tion comprising aluminum powder in an aqueous 801w 29/195 197 tion of a combination of phosphoric acid, chromic acid, or molybdic acid, or metal salts of said acids. [56] References Cited UNITED STATES PATENTS 1/1961 Drummond 117/71 9 Claims, No Drawings PROTECTIVE COATING FOR FERROUS METALS This is a continuation, of application Ser. No. 149,360 filed 6/2/71, now abandoned.

The present invention relates to the protective coating of ferrous metals to prevent corrosion thereof, and relates particularly to the protective coating of steel to inhibit corrosion and stress corrosion cracking. The invention further relates to such protected ferrous metals.

The corrosion of ferrous metals in corrosive atmospheres is a problem that has long faced the metal working arts. The problem of preventing corrosion is particularly acute in the manufacture of metal fasteners, such as bolts, where corrosion of the fastener when under stress can result in stress corrosion cracking and sudden failure of the fastener. In high-strength fasteners used in the aerospace industry, the problem of stress corrosion cracking and failure are particularly troublesome. While stress corrosion cracking in metals is not limited to steels, ultra-high strength steels are more susceptible than most other alloys because of their highly stressed condition and low load bearing area after pit initiation. Nevertheless, the use of ultrahigh strength steels for fasteners in the aerospace industry is still widespread because of their low initial cost.

There have been prior attempts made in the art to prevent corrosion and stress corrosion cracking of ferrous metals, such as the ultra-high strength steels, by plating the metals (or articles such as bolts made therefrom) with more noble metals.

Thus, articles such as fasteners made of metals susceptible to corrosion and stress corrosion cracking have been nickle-plated. The plated metal, which is more noble than steel, is relatively inert and of low reactivity with corrosive materials in the environment. Nevertheless, the use of nickel coatings alone has not solved the problem of corrosion. Since the nickel coating is protective only where deposited, any failure in the coating, such as a pinhole, means that the metal beneath the failure is not protected. Thus, to insure adequate protection, very thick layers of nickel (greater than about 0.5 mil) are required. Not only is the cost of such thick protective layers of nickel on a substrate prohibitive, but deposition of the nickel coating by electroplating, the most feasible method, becomes extremely difficult. In electroplating, the risk is run that hydrogen codeposited with the plated metal will be adsorbed and result in hydrogen-embrittlement of the un derlying ferrous metal. This risk increases as the thickness of the nickel coating electroplated is increased, since the hydrogen is not easily baked out through thick coatings.

Further, the electroplating of fasteners with a metal such as nickel in thicknesses greater than about 0.5 mil is not feasible because the plated layer is unevenly deposited. If a bolt is electroplated, for example, the thread roots are areas of lower current density than the thread crests, leading to a preferential deposit of the plating metal on the crests. This may cause difficulties in mating the bolts with nuts, particularly if the nuts have been similarly electroplated.

A further objection to nickel coatings, particularly in fastener applications, is the tendency of the metal to seize and gal].

It has been attempted in the art to overcome these deficiencies of pure nickel coatings by the use of coatings combining a metal more noble than iron (such as nickel) with a metal less noble than iron. Thus, ferrous articles such as fasteners made of ultra-high strength steels have been coated with an alloy of nickel and cadmium (so called diffused nickel-cadmium" coatings) by first plating the article with nickel, then overplating with cadmium, and heating at about 650F. to diffuse and alloy the metals. In such coatings, the cadmium acts as a sacrificial metal which is corroded preferentially with respect to the ferrous metal under corroding conditions.

The drawback of such nickel-cadmium coatings is that they lose their sacrificial property toward underlying and surrounding base metals at relatively low temperatures, i.e. at temperatures between 650 900F., although temperatures within this range are commonly encountered in aerospace applications, for example in certain parts of jet engines. When diffused nickelcadmium protective coatings are heated to these temperatures, the nickel apparently diffuses upwardly through the coating, making the surface of the coating more noble and less sacrificial.

Also, under high temperature conditions, cadmium can escape from the plating and may cause stress-alloy cracking, either of the metal structure surrounding a plated article such as a bolt, or of the article itself if the nickel undercoating is defective. Under these circumstances, cadmium acts as a corrosive medium attacking the grain boundaries of the metal with which it alloys.

According to the present invention, a protective coating is described which is highly effective in preventing corrosion and stess corrosion cracking in ferrous metals, particularly high strength steel, to which the coating is applied. The coatings are of particular utility when employed to protect articles exposed to high temperatures and stresses, e.g. ultra-high strength steels fashioned into components for jet engines or metal fasteners used in aerospace applications. The protective coating comprises a first coating of a metal more noble than the ferrous metal to be protected and a coating of phosphate chromate bonded aluminum (PCBA) over said first coating. These coatings provide sacrificial protection to metals with which the protected metal is in contact, and resist corrosion and stress corrosion cracking of the metal itself. The first property is of particular importance in the aerospace industry where steel fasteners are often employed in contact with surrounding aluminum structures which, because they are of a metal more active than the metal of the fastener, are often undesirably corroded. The PCBA coating is more sacrificial than the diffused nickel-cadmium alloy coatings heretofore used in the art and is free of cadmium which could cause stress-alloy cracking. More importantly, the PCBA coating remains sacrificial even after exposure to elevated temperature conditions under which nickel-cadmium coatings would lose this protective property, as hereinbefore discussed.

As the noble metal of the first coating any metal more noble than the ferrous metal being coated can be employed, including tin, tantalum, titanium, copper, nickel, cobalt and the conventional noble metals or precious metals such as silver, gold, platinum, palladium, etc. The use of precious metals is impractical because of cost, however, Metals such as tantalum and titanium cannot be applied as coatings by electroplating and their use as coatings is, thus, inconvenient and- /or expensive. A metal such as copper may tend to form intermetallic compounds with a ferrous substrate at high temperatures after long periods of time. The noble metals most practical for use are nickel and cobalt. Those metals are relatively inexpensive, can be electroplated, and are particularly suitable for use at high temperature. Nickel is presently preferred as the plating metal because of its lower cost, and is shown in this specification as the preferred embodiment. However, the cost of this metal is increasing to values at which cobalt, with properties comparable with those of nickel, is becoming economically competitive.

The phosphate chromate bonded aluminum materials employed according to the present invention are described and claimed in US. Pat. No. 3,248,251, where their use as protective coating and bonding compositions for metals is taught. However, the use of such coatings alone on ferrous metals such as steels subject to stress corrosion cracking is less satisfactory than when used as an overcoating on a noble metal. While the PCBA coating remains sacrificial at high or low temperatures, it is not a moble material (with respect to ferrous metals) and cannot per se prevent pitting. Such a coating used alone merely delays failure of the underlying ferrous alloy.

As described in the aforementioned patent, the teachings of which are incorporated herein by reference, the PCBA materials employed together with a noble metal according to the present invention to form a protective coating on ferrous metals comprise an aqueous dispersion of metallic aluminum having a grain size less than 325 mesh in an aqueous solution of a combination of inorganic compounds from the group consisting of phosphoric acid, chromic acid, molybdic acid, and metal salts of said acids. The solution of inorganic materials comprises at least one mol per liter of dissolved phosphate, at least 0.3 mol per liter of dissolved chromate or molybdate, and at least 0.5 mol per liter of dissolved metal such as magnesium, zinc, aluminum, iron, calcium, lithium, sodium, silver, etc. The suspended aluminum powder is present in an amount of from about to 2000 grams per liter of solution. As further taught in the patent, the composition is commonly applied by conventional techniques such as spraying, dipping, rolling, or brushing to the surface to be coated and is subsequently cured to waterinsolubility by heating at a temperature of about 500 1000F. for periods of time variable with the temperature employed, but generally at least about minutes.

It has been found that lower temperatures than those mentioned in this patent can be used for curing the coatings. For example, heating at a temperature of about 375F. for 24 hours will cure the coatings to water insolubility. The use of low cure temperatures is particularly desirable when coating threaded fasteners with PCBA, since exposure to high temperatures causes a loss of compressive stresses introduced in the threads by rolling. The maintenance of these stresses is, however, desirable since they increase the fatigue life of the metal.

Any ferrous metal can be protected from corrosion according to the present invention, including stainless steels and those with high nickel content. However, the problem of stress corrosion cracking is most important, as discussed above, in high alloy steels. Such high alloy steels include those known in the art as H-1 1, A15] 4340, AlSl 4130, the maraging-type steels such as Maraging 300, and, in general, any ultra-high strength alloys having a minimum ultimate tensile strength of at least about 220,000 psi.

For preparing the protective coatings of the present invention, the surface to be coated is cleaned, preferably by a technique such as dry blasting with particles of alumina. in general, pickling of the alloys for purposes of cleaning is to be avoided to preclude hydrogenembrittlement of the alloy which may result from treatment with acid.

The metal to be protected is then coated with the noble metal by an suitable method such as electroplating vacuum deposition or the like. The thickness of the noble metal is between about 0.1 mil and about 0.4 mil, preferably between about 0.1 mil and about 0.3 mil when fasteners are being coated. When coating with nickel, electroplating from a sulfamate bath is particularly suitable.

To inhibit hydrogen-embrittlement, electroplated articles are then preferably heated to drive off any hydrogen codeposited during the electroplating step. Such heating may be for a period between 2 and 24 hours at moderate temperatures, for example about 375F.

The coated surface may next optionally be lightly blasted (for example with alumina particles) to toughen it and thus to improve the physical adhesion of the PCBA coating to be applied thereover. Before or after blasting, the parts may also be subjected to degreasing in an alkaline bath, particularly if they have been stored for any length of time.

Whether the coated surface has been previously roughened or not, the PCBA solution is next applied in an amount sufficient to give a cured coatiing at least about 0.3 mil thick. When coating fasteners, the coating is preferably between about 0.3 mil and about 0.5 mil thick, but for other articles an upper limit on coating thickness may be dictated only by economy, and the coating may be several mils thick. Even with a sacrificial PCBA coating of this thickness, the noble metal undercoating serves important functions, e.g. as a noncorrosive barrier which also prevents diffusion of aluminum into the ferrous substrate at high temperatures. To permit easier control of thickness, the PCBA coating is preferably applied by spraying.

After application of the PCBA, the coating is preferably gently heated slowly to drive off water and to render the coatings dry to the touch. This drying prior to curing is optional, but is convenient. For example, after such drying, coated articles (such as bolts) may be stacked without damage. For this step, drying at temperatures between 200F., preferably about F., for about 10-20 minutes is sufficient.

Subsequently, the coating is cured to waterinsolubility by heating at higher temperatures to remove any remaining water and to drive off any hydrogen which may have been generated by reaction of the metals with the acid components of the bath. As mentioned earlier, such curing may be at temperatures as low as 350 375F. for 24 hours or more, or may be accomplished in as little time as 15 minutes at 650F., for example.

The electrical conductivity of the cured coatings can be improved to make then more capable of providing galvanic protection to surrounding structures by heating them at temperatures of 850 to 1000F. for a period of at least 1 1 /2 hours. If exposure of the coated article to high temperatures is undesirable, for example for the reasons discussed earlier herein in the case of threaded fasteners, conductivity in the coatings can alternatively be increased by mechanically working the coatings, for example by blasting or burnishing. Suitably, the coatings may be mechanically worked by blasting with glass beads (e.g. 150 200 mesh size), or can, alternatively, be burnished by a fine wire wheel. Wheels comprising stainless steel wires are preferred to preclude any possible embedment of corrodable wire particles in the coatings. By mechanical working, nonconductive non-metallic layers over the aluminum particles in the coating are apparently broken and contact between the particles establishing an electrically conductive path therebetween is improved.

The coated articles are now ready for use, or still further coatings may be applied thereover, such as conventional conversion coatings.

A better understanding of the present invention and of its many advantages will be had by referring to the following specific examples, given by way of illustration.

EXAMPLE 1 Bolts of 11-11 ultra-high strength steel, and of B5F5 and 418 stainless (both high strength steels) were each coated with a PCBA coating alone, with nickel and PCBA (according to the present invention), and with a diffused nickel-cadmium coating.

More specifically, the PCBA-coated samples included samples in which the coating was: (1) cured at 375F. for 24 hours and then burnished with glass beads; (2) cured at 650F. for 15 minutes and then burnished with a wire wheel; and (3) cured at 650F. for 15 minutes and then at 1000F. for 90 minutes. The PCBA coatings were between 0.3 and 0.5 mil thick.

The samples of PCBA on nickel all had PCBA coatings from 0.3-0.5 mil thick but different samples had nickel plating either 0.1, 0.2, or 0.4 mil thick: in each case the PCBA coating thereover was cured at 1000F. for 90 minutes.

The diffused Ni-Cd samples had 0.10.2 mil cadmium deposited on 0.20.4 mil nickel. Diffusion was effected by heating at 630F. for one-half hour.

The bolt samples were first exposed to a temperature of 900F. for 8 hours, followed by a 16 hour exposure to 5 percent salt spray. This treatment (one cycle) was repeated for 20 cycles, with color photographs being made on the coatings after 5, l0, l5, and 20 cycles. The observations are summarized in Table 1 below.

EXAMPLE 2 TABLE 11 Coating Hours to Failure PCBA 667 SF 1128 SF 886 SF (9 samples) 604 SF 1267 SF 1467 SF 456 SF 1300 SF 1488 SF Diffused Ni-Cd 1769 SF 2403 NF) testing (6 samples) 2403 NF) halted 2403 NF) PCBA-Ni 1464 SF 2246 SF (6 samples) 2384 SF 2614 SF 2567 SF 3571 SF Uncoated H-1 1 94 SF 457 SF (6 samples) 101 SF 144 SF 104 SF 138 SF What is claimed is:

1. The method of protecting ferrous metal to inhibit corrosion which comprises coating said ferrous metal with a layer, from about 0.1 mil to about 0.4 mil thick, of a more noble metal, and then coating said more noble metal with a sacrificial layer, at least about 0.3 mil thick, by applying thereto a composition consisting essentially of about 10 to 2000 grams of alumiunum powder, having a grain size less than 325 mesh, per liter of an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or salts of said acids, and then heat curing said composition at a temperature from 350 to 1000F. for a time between at least 24 hours at the lowest temperature in the range specified to about 15 minutes at the highest temperature in the range specified, until said composition is water-insoluble.

2. The method as in claim 1 wherein said sacrificial layer, after heat curing to water-insolubility, is mechanically worked, whereby its electrical conductivity is increased.

3. The method as in claim 1 wherein said sacrificial layer, after heat curing to water insolubility, is heated at a temperature from 850 to 1000F. for at least 1 /2 hours, whereby its electrical conductivity is increased.

4. The method as in claim 1 wherein said layer of more noble metal is an electroplated layer of nickel or cobalt.

5. The method as in claim 1 wherein said ferrous metal is steel.

6. The method of inhibiting stress corrosion cracking in a fastener of high-alloy steel which comprises coating said fastener with nickel in a thickness from about 0.1 mil to 0.3 mil, then coating said nickel by applying thereto a composition consisting essentially of about 10 to 2000 grams of aluminum powder, having a grain size less than 325 mesh, per liter of an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or salts of said acids, and then heat curmethod of claim 1.

8. A protected ferrous metal article prepared by the method of claim 2.

9. A protected ferrous metal article prepared by the method of claim 3. 

1. THE METHOD OF PROTECTING FERROUS METAL TO INHIBIT CORROSION WHICH COMPRISES COATING SAID FERROUS METAL WITH A LAYER, FROM ABOUT 0.1 MIL TO ABOUT 0.4 MIL THICK, OF A MORE NOBLE METAL, AND THEN COATING SAID MORE NOBLE METAL WITH A SACRIFICIAL LAYER, AT LEAST ABOUT 0.3 MIL THICK, BY APPLYING THERETO A COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 10 TO 2000 GRAMS OF ALUMIUNUM POWDER, HAVING A GRAIN SIZE LESS THAN 325 MESH, PER LITER OF AN AQUEOUS SOLUTION OF A COMBINATION OF PHOSPHORIC ACID, CHROMIC ACID, OR MOLYBDIC ACID, OR SALTS OF SAID ACIDS, AND THEN HEAT CURING SAID COMPOSITION AT A TEMPERATURE FROM 350* TO 1000*F. FOR A TIME BETWEEN AT LEAST 24 HOURS AT THE LOWEST TEMPERATURE IN THE RANGE SPECIFIED TO ABOUT 15 MINUTES AT THE HIGHEST TEMPERATURE IN THE RANGE SPECIFIED, UNTIL SAID COMPOSITION IS WATER-INSOLUBLE.
 2. The method as in claim 1 wherein said sacrificial layer, after heat curing to water-insolubility, is mechanically worked, whereby its electrical conductivity is increased.
 3. The method as in claim 1 wherein said sacrificial layer, after heat curing to water insolubility, is heated at a temperature from 850* to 1000*F. for at least 1 1/2 hours, whereby its electrical conductivity is increased.
 4. The method as in claim 1 wherein said layer of more noble metal is an electroplated layer of nickel or cobalt.
 5. The method as in claim 1 wherein said ferrous metal is steel.
 6. The method of inhibiting stress corrosion cracking in a fastener of high-alloy steel which comprises coating said fastener with nickel in a thickness from about 0.1 mil to 0.3 mil, then coating said nickel by applying thereto a composition consisting essentially of about 10 to 2000 grams of aluminum powder, having a grain size less than 325 mesh, per liter of an aqueous solution of a combination of phosphoric acid, chromic acid, or molybdic acid, or salts of said acids, and then heat curing said composition at a temperature from 350* to 1000*F. for a time between at least 24 hours at the lowest temperature in the range specified to about 15 minutes at the highest temperature in the range specified, until said composition is water-insoluble, and then mechanically working said heat cured layer.
 7. A protected ferrous metal article prepared by the method of claim
 1. 8. A protected ferrous metal article prepared by the method of claim
 2. 9. A protected ferrous metal article prepared by the method of claim
 3. 